This application relates to a refrigerant superheat control to enhance system performance and improve compressor reliability.
In air conditioning, heat pump and refrigeration systems, a superheat of the refrigerant leaving an evaporator needs to be closely controlled. Refrigerant leaves the evaporator normally at the superheated state, where its actual temperature is higher than the corresponding saturation temperature (a superheat is actually defined as the difference between these two temperatures). A certain (positive) superheat is typically required to ensure that little or no liquid refrigerant enters the compressor and system operation is stable. If a significant amount of liquid refrigerant enters the compressor, an undesirable condition known as “flooding” will occur.
On the other hand, it is known that in order to assure the highest performance (efficiency and capacity) of the refrigerant system, close to zero superheat values for the refrigerant leaving the evaporator are to be maintained. Further, by reducing suction superheat, the oil return to the compressor is also improved, as the oil viscosity is reduced with the reduced superheat. This is true, since more refrigerant is diluted in the oil at lower superheat values. Conversely, as the superheat value is increased, refrigerant is boiled off from the oil increasing the oil viscosity and making the oil more prone to stagnate at the evaporator exit or in the piping connecting the evaporator to the compressor. Of course, improving oil return is a goal of a refrigerant system designer, as it enhances compressor reliability and enhances system performance by preventing oil retention in the evaporator and associated piping.
While it is known to be desirable to reduce the superheat to the lowest value possible, to date most refrigerant system, at best, would operate with superheat values in a range of 6-12° F. The potential for a measurement error due to temperature sensor measurement tolerances, calibration and resolution; system component manufacturing variability; ambient effects on system operation; load demand fluctuations and associated transient phenomena, concurrently occurring within the refrigerant system, have typically provided a practical bar to further reduction in the superheat setting.
As also known, typically, a temperature (and the associated superheat value) of the refrigerant downstream of the evaporator is utilized for the system operational control either to provide safe and reliable compressor operation, or to prevent an expansion device, such as a thermostatic expansion valve, malfunctioning, or both.
It is undesirable, as mentioned above, to have significant flooding in the compressor, due to associated reliability issues. Thus, the refrigerant system designers have erred on the side of applying sufficient superheat to eliminate any potential for such flooding at an entire spectrum of operating conditions. Uncontrolled flooding results in a drastic drop in compressor capacity and efficiency, and may also cause severe damage to the compressor.
The present invention allows operation at a much lower superheat setting, and perhaps even with slight flooding at the compressor entrance (or evaporator exit), without any detrimental effects on compressor reliability and at higher system efficiency and capacity. At the same time, the present invention ensures that no significant amount of liquid refrigerant will enter the compressor pumping elements.